Feeding ecology of forage fishes in the San Juan Archipelago: Diet composition and variation in Ammodytes hexapterus, Hyperprosopon ellipticum, and Clupea pallasii

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Abstract

Forage fish play an important role in wasp-waist ecosystems like that of the San Juan
Archipelago. These small fish, including the Pacific sand lance, silver surf perch, and
Pacific herring, regulate the flow of energy through the trophic levels of this marine
ecosystem, from the plankton to the larger predatory fish, sea birds, and marine
mammals. Gut content analyses reveal that Pacific sand lance and Pacific herring feed
primarily on calanoid copepods, while the surf perch has a more varied diet. Pacific sand
lance and Pacific herring had low values on Levins’ standardized measure of niche
breadth, indicating a narrow dietary breadth characteristic of a specialized diet. Surf
perch had a higher value, indicating a wider dietary breadth and more generalist feeding
approach. Morista’s measure of niche overlap was used to quantify dietary overlap
between the species. Pacific sand lance and Pacific herring had a high rating, which could
indicate competition for the same prey items. Both of these species’ dietary overlap with
surf perch was much lower, suggesting little competition for prey items. The diet
composition of surf perch exhibited significant temporal variation. The diet composition
of Pacific sand lance did not exhibit significant temporal variation, but did exhibit
significant spatial variation between Jackson Beach and the sand wave channel. By
quantifying these measures of compositional, temporal, and spatial variation in forage
fish diets, we can now better understand the types of resource partitioning that are
influencing trophic exchange in the San Juan Archipelago.